Chromatographic separation mediating

For initial work on cyclodextrin-mediated chromatographic separation of enantiomers, see Hinze, W. L. Armstrong, D. W. ed. Ordered Media in Chemical Separations, ACS Symposium Series 342, 1986. 

Cyclodextrin-mediated chromatographic separation of enantiomers is also discussed in Braithwaite, A. Smith, F. J. Chromatographic Methods, 5th Edition, Blackie Academic Professional, London, New York, 1996. 

Transition metal mediated or catalyzed benzene formation reactions have been reported using various metals. However, the use of three different alkynes is difficult [38], In many cases, a mixture of several benzene derivatives is obtained. In 1974, Wakatsuki and Yamazaki used three different alkynes with Co complexes [27b], but isomers were formed and a tedious chromatographic separation was necessary. The first selective coupling of three different alkynes in high yields was reported in 1995 using a combination of unsymmetrical zirconacydopentadienes and CuCl, as shown in Eq. 2.52 [7k]. 

Interactions even weaker than ionic bonds can be used to separate enantiomers. Chromatographic separation relies on a difference in affinity between a stationary phase (often silica) and a mobile phase (the solvent travelling through the stationary phase, known as the eluent) mediated by, for example, hydrogen bonds or van der Waals interactions. If the stationary phase is made chiral by bonding it with an enantiomerically pure compound (often a derivative of an amino acid), chromatography can be used to separate enantiomers. 

Surfactant-Mediated Chromatographic Separations. The selective interaction of surfactants with a variety of solutes (as ion pairs with monomeric surfactant molecules or as bound ("associated") species... 

In common with other application areas of chromatographic separation, a considerable amount of effort has been expended recently on the development of different elution conditions and types of stationary phases for peptide separations in attempts to maximize column selectivities without adversely affecting column efficiences. Peptide retention will invariably be mediated by the participation of electrostatic, hydrogen bonding, and hydrophobic interactions in the distribution phenomenon. The nature of the predominant distribution mechanism will be dependent on the physical and chemical characteristics of the stationary phase as well as the nature of the molecular forces which hold the solute molecules within the mobile and stationary zones. The retention of the solute in all HPLC modes can be described by the equation... 

Another approach to enantiomerically pure planar chiral azaferrocenes involves 2-lithiation of (367) followed by addition of (-)-menthyl-(5 ) — jo-toluenesulfinate. The diastereomeric sulfoxides thus obtained are chromatograph-ically separable, and treatment of each diastereomer with t-BuLi produces an enantiomerically pure planar chiral anion that may be trapped with an electrophile (Scheme 98). Finally, in order to obviate the need for performing a resolution or a chromatographic separation, chiral ligand-mediated enantioselective deprotonations have been investigated. Lithiation of (367) in the presence of (-)-sparteine followed by addition of an electrophile gives the 2-substituted azaferrocene in good enantioselectivities (Scheme 99). However, lateral lithiation of (370) mediated by 5-valine-derived bis(oxazoline) (371) provides planar chiral products with excellent enantios-electivity. 

DCC-mediated condensation of hydroxy aldehyde 240 and acid 268 gave the aldehyde ester 269 (70%). An intramolecular olefination followed by chromatographic separation of the desired C-8 diastereomer led to the isolation of the 16-membered ring macrocycle 270 in 20% yield. Desilylation, oxidation, and cyclization then afforded a 47% yield of y-lactone 271. Acylation of the C-3 hydroxyl group was followed by reduction of the y-lactone and C-9 carbonyl. 

Taylor, W.G., D.D. Vedres, and J.L. Elder. 1993. Capillary gas chromatographic separation of some diastereomeric amides from carbonyldiimidazole-mediated microgram-scale derivatizations of the acid moiety of permetJirin insecticide. /. Chromatogr. 645 303-310. 

Two interesting features of the synthesis worthy of some discussion are the addition of an organometallic reagent to aldehyde 584 and the choice of the protecting group. The zinc-mediated allylation of 584 produces a chromatographically separable 71 29 mixture of anti diastereomer 585 and the corresponding syn diastereomer (not shown). This stereoselectivity... 

The limitations of ion exchange materials for lanthanide separations based on the aquo cations led to the development of separation procedures mediated by aqueous complexants. The first such separations used ammonium citrate as the eluant. The displacement of from the resin by and NHJ is greatly augmented by the formation of lanthanide-citrate complexes, which tend to enhance transfer of the lanthanide ions to the mobile phase. The relative rates of movement of the rare-earth cations down the column is thus impacted not only by the affinity of the resin phase for the cations, but also by the relative stability of the aqueous citrate complexes. This approach forms the basis of the most useful and successful chromatographic separations of the lanthanides. 

The synthesis of 1 by a zinc-mediated ring closure of 17 resulted in a process applicable for a multi purpose production plant. The new procedure reduced e number of isolations from 6 to 4. It eliminated the use of the hazardous reagents, ethyleneoxide and butyllithium, low reaction ten5>eratures and chromatographic separation. Furthermore, the overall isolated yield was improved from 18 to 29%. 

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